Your browsing history can be the reason why you've received such error message. Thus, let's perform some browser troubleshooting steps so we can get rid of it successfully.
To start with, let's begin by accessing your account in a new incognito or private window of your browser. This way, we can check if this error is related to your current browser or not. To do so:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once completed, you can log back into your account and access your files. If you can do so without any errors, you can clear your browser's cache to optimize it. Also, your browser can start from scratch and improve your QuickBooks Online browsing experience.
However, if you still received the same error message, let's switch and use a new browser application. Then, log back into your account and access your files from there.
Know that you can always visit our Help Articles for QuickBooks Online in case you need some tips and related articles for your future tasks.
If there's anything else that I can help you with, please let me know in the comment section down below. I'll be always around ready to help.